1. FIELD OF THE INVENTION
This invention relates to an invertor device for AC power units, and more particularly to an invertor device of the pulse width-modulation type for use in portable AC power units.
2. PRIOR ART
In recent years, portable AC power units have been increasing in number which are equipped with invertor devices for stabilizing the output frequency. For example, a portable power unit which generates AC power having a commercial frequency by means of an AC generator driven by an engine is known, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 59-132398, in which the engine is operated within a high speed range to cause the AC generator to generate high AC output, the output is converted into direct current, then the direct current is converted into alternating current by an invertor device, and the alternating current is outputted.
There is a demand for AC power units of this kind whose output waveform is as close to a sine wave dependent upon their application as possible. To this end, an attempt has been made to employ an invertor device of the pulse width-modulation type as the above-mentioned invertor device (Japanese Provisional Patent Publication (Kokai) No. 60-82098).
However, if such an AC power unit is started to operate with a relatively large load connected thereto, starting current can temporarily flow in a large amount to exhibit too large a peak current, which is liable to adversely affect the operation of the AC power unit per se. Therefore, there is also a demand for an AC power unit capable of cutting off such too large a peak current. However, frequent cutoff of the supply of the output AC power will cause inconveniences in actual use.
Further, it is preferable that circuit protection of the AC power unit can be quickly performed also when the load is temporarily short-circuited.
Loads connected to the AC power unit can be of various types having different load characteristics. For instance, if a load having a half-wave rectification characteristic is connected thereto, overload can occur during a half cycle of each cycle of AC power output. In this case, detection of an average output current is not sufficient to ensure proper circuit protection of the AC power unit. If a threshold value indicative of an upper limit of load applicable to the unit for the circuit protection is set to a low level so as to obtain circuit protection in the case of the half-wave rectification load, even a normal current which flows e.g. when a full-wave rectification load is connected, can be regarded as an overcurrent, to improperly effect circuit protection.